The present invention relates to a power supply system utilizing a 3 phase power source in which line current distortion is diminished.
FIG. 1 illustrates a circuit of a conventional power supply system and FIG. 2 wave forms at respective portions thereof. Referring to FIG. 1, with a full wave rectifying bridge consisting of diodes D.sub.1 . . . D.sub.6 linked to a 3 phase power source, the AC voltages are converted into a DC voltage V.sub.DC and, then this DC voltage is introduced into an inverter 2 for inversion into a high frequency wave, which is applied to a load 3.
FIG. 3 shows an exemplary circuit of the inverter 2, in which an oscillator circuit is composed of transistors Q1 and Q2 and a transformer T. The load 3, comprises any equipment which utilizes AC power, e.g., motor, etc. In some instances, the RPM of the motor is controlled by altering the oscillation frequency or output voltage of the inverter 2. For equipment which uses DC power, the inverter 2 may be omitted. In such a power supply system, since the 3 phase alternating currents at the input are subjected to full wave rectification by diodes D.sub.1 . . . D.sub.6, comparisons between the input voltages of respective phases are made. Thus, considering the half cycle of a phase voltage, power is delivered to the load only for the range of 2/3 when the voltage is higher than in the other two phases. As a result, the line current wave form I from the 3 phase power source takes the form of a quiescent 1/3.pi. radian period and conducting 2/3.pi. radian periods during the half cycle, as shown in FIG. 2. The distortion factor of this current I, as actually measured, shows as high a value as 27%. A large distortion factor means a large high harmonic component, which naturally results in increased loss in the transformer. In the 3 phase power source, the secondary output is delta-connected which causes a short-circuiting current caused by the third high harmonic to flow, thereby further increasing loss. For this reason, the efficiency of the power supply transformer declines, permitting only a smaller power than initially designed to be delivered to a load. Conversely, to achieve a desired power output requires a power supply larger than is necessary, which is uneconomical. Since the loss which occurs inside the transformer is mostly converted into heat, the service life of the transformer is shortened. If the transformer is designed for higher heat resistance, an increase in cost results.
As an alternative, a power supply system like that described in U.S. Pat. No. 4,143,414 has been proposed. In this system, to each phase of the 3 phase AC power source, is joined an AC-DC converter composed of a first rectifier, a DC-AC converter and a second rectifier, all connected in series, and the output is delivered by connecting the DC ouputs from respective converters in parallel. Even in this system, voltage comparison is performed because respective second rectifiers are connected in parallel at the final DC output stage. Consequently, an input current quiescent period is produced on the 3 phase AC side, resulting in a higher distortion factor. Thus this system involves a similar deficiency as was involved in the prior art power supply system first described.
The present invention has been designed to overcome the above-mentioned difficulty. Thus, an object of the invention is the provision of a power supply system which obtains at its output a high frequency power with nearly constant amplitude. This is accomplished by inverting the power in a plurality of AC phases into a high frequency power and superposing each in series. In this way, the line current distortion of the 3 phase power source is greatly reduced, thereby attaining smaller loss and a reduction in overall size of the system and higher operating efficiency.